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A device for the utilisation of wave energy and a method

The present invention relates to a device for the utilisation of wave energy, which device comprises

- a Darrieus rotor having at least two Darrieus rotor blades, and - a Wells rotor having at least two Wells rotor blades, wherein the Darrieus rotor and the Wells rotor are rotatable about a common axis of rotation A.

Such a method is known from WO02/44558. Herewith, wave energy can effectively be harnessed. The advantage of wave energy, namely a high energy density, also presents a problem for devices that are used for the utilisation of wave energy. Because they have to be able to withstand heavy weather conditions, they are relatively expensive. Therefore, it is required that a device for the utilisation of wave energy can harness as much energy as possible.

The object of the present invention is to provide an improved device whose energy efficiency is increased considerably, thus reducing the costs of the energy.

To this end the present invention provides a device which com-prises

- a Darrieus rotor having at least two Darrieus rotor blades, wherein the Darrieus rotor has a solidity σD, and

- a Wells rotor having at least two Wells rotor blades, wherein the Wells rotor has a solidity σW, wherein

- the Darrieus rotor and the Wells rotor are rotatable about a common axis of rotation A, and

- σW is larger than or equal to σD.

Experiments with such a device in a wave test tank have sur-prisingly shown that such a device can convert wave energy into mechanical energy (and thus also into hydraulic energy, electricity and optionally subsequently also into hydrogen) with an efficiency that is larger than the sum of i) the energy harnessed using a Darrieus rotor only, and ii) the energy harnessed using a Wells rotor only. Although it is possible to have the σW meet the stipulated conditions by markedly increasing the number of Wells blades, in practice it will be preferred to broaden the Wells rotor blades, and this number is preferably equal to the number of Darrieus blades or a multiple thereof, such as 3 or preferably 2 times as much. These latter cases are in fact favourable when the Darrieus blades are connected by 3 or 2 Wells blades to an axle (that coincides with the axis of rotation) . The ratio of i) the overall length of the effective blade length of the blades of the Wells rotor, and ii) the overall length of the effective blade length of the blades of the Darrieus rotor will usually lie between 0.01 and 3.0, such as between 0.1 and 2.5 and preferably between 0.5 and 1.5.

The application uses the following definitions:

A rotor is an assembly of two or more rotor blades, the latter also simply being referred to as blades.

A Wells rotor comprises at least two blades, wherein the blades are convex at both sides of a plane defined by the leading edge and the trailing edge of the blade. Relative to that plane a Wells blade is preferably mirror symmetrical. Wells blades predominantly extend in a radial direction relative to the axis of rotation of the Wells rotor. The plane of a blade can be at an angle with the normal of at most 15° to the axis of rotation of the Wells rotor, preferably at most 5°, and even more preferably 0°.

The solidity of a Wells rotor can be calculated using the formula N.Awb / Aw wherein . N = the number of blades

Awb = the surface area of a Wells rotor blade (for a rectangular blade this is the chord width c x the Effective blade length L) Aw = the Effective swept surface area of the Wells rotor. If the blades • differ from each other, then the above formula has to be applied to each of the blades, and the separately obtained outcomes of all blades have to be added up.

A Darrieus rotor comprises at least two blades, wherein the blades in the plane of the axis of rotation and the normal to the axis of rotation in said plane are at an angle with the axis of rota-tiόn of at most 60°, and preferably at most 45°. A favourable angle is for instance 0° (wherein a Darrieus blade does not intersect the axis of rotation) . Preferably, the lower distal end of a Darrieus blade is situated closer to the axis of rotation of the device than the upper distal end. In that case an angle between 25 - 35° is most preferred.

The solidity of a Darrieus rotor can be calculated using the formula = N.Adb / Ad wherein

N = the number of blades

Adb = the surface of a Darrieus rotor blade (for a rectangular blade this is the chord width c x the Effective blade length L)

Ad = the Effective swept surface area of the Darrieus rotor. If the blades differ from each other, then the above formula has to be applied to each of the blades, and the separately obtained outcomes for all blades have to be added up.

Solidity can be understood easiest for a wind turbine, where the direction of the flow of medium (i.e. air) runs parallel to the axis of rotation (and perpendicular to the plane in which the blades rotate) of the wind turbine.

The above solidity formulas are basically equal, but for the Wells rotor the surface area Aw is determined as if (in case of a vertical axis of rotation) the flow direction of medium (i.e. water) were parallel with the axis of rotation. For a Darrieus rotor the surface area Ad is determined (in case of a vertical axis of rotation) as if the flow direction of medium were perpendicular to the axis of rotation. For the interested layman who is not familiar with the term solidity, a few examples have been given in the description of the drawings.

The width (also referred to as chord) of a blade is the shortest crossing distance between the front side and back side of the profile (in English the leading edge and the trailing edge) .

The term "effective" in connection with Wells and Darrieus blade length, means that only to the extent where a respective Wells or Darrieus effect is present, the blade surface in question is taken into account. In other words, only insofar as the part of the blade surface in question contributes to the generation of energy. The ordinary person skilled in the art will not need further elucidation. In order to achieve a maximum energy output, a device according to the invention will preferably be dimensioned based on the expected wave pattern in the body of water. In that case the maximum diameter will preferably be smaller than one third, more preferably be smaller than one fourth of the wave length prevailing in the particular sea or ocean.

For even better results σW is at least 15% larger than σD, and preferably at least 25% larger. In practice, a value above 200% will not readily be opted for. In practice σW will most preferably be between 30% and 100% larger than σD.

Preferably, at least one Wells rotor blade is directly connected to a Darrieus rotor blade.

This further increases the efficiency of the generation of en-ergy.

This is even more so when the distal end of the Wells rotor blade is connected with the Darrieus rotor blade.

In practice, a typical device according to the invention will contain a generator selected from i) a generator for generating elec-tricity, and ii) a generator for generating hydraulic pressure.

The present invention also relates to a method for harnessing energy by the utilisation of wave 'energy, wherein a device according to the invention is placed in a body of water in which waves occur naturally. The average wave height (between trough and crest) of any such body of water is at least 50 cm per year.

A preferred embodiment is characterized in that the Wells rotor blades are situated at a depth between 0.5 and 2.0, preferably between 0.8 and 1.25 times the 5-minutes' average wave height below the level of the body of water. ' Thus, a high energy output can be achieved.

Preferably, the upper ends of at least two Darrieus blades extend to above twice the year-average wave height.

Thus, a high energy output can be achieved.

Preferably, the 5-minutes' average of the axis of rotation is in an orientation of less than 5° to the vertical.

In that case, the highest energy output is achieved.

Preferably, energy is selected from hydraulic energy, electricity or hydrogen gas is generated.

The hydrogen gas can be obtained by means of electrolysis with electricity generated using wave energy.

The present invention will now be illustrated by the drawings, in which fig. 1 shows a perspective view of a device for the utilisation of wave energy according to the invention; fig. 2 shows a top plan view of a detail of the device of fig. 1; fig. 3 shows a variation of fig. 1, wherein the device has obliquely arranged Darrieus blades; fig. 4a-d show graphs of measurements illustrating the increased efficiency of a device for the utilisation of wave energy according to the invention; fig. 5 - 8 show a number of devices according to the invention in order to elucidate the term solidity.

Fig. 1 shows a device according to the invention for the utilisation of wave energy, which device has three first rotor blades 101 of the Darrieus type and three second rotor blades 102 of the Wells type. The second rotor blades 102 are at their distal ends attached to a central axle 104, which is connected to a generator 105 for generating electricity. The device shown in fig. 1 is placed into the sea for instance by using a pillar (not shown) , as described in the earlier application WO02/44558. Fig. 2 shows a top plan view of the assembly of the rotor blades 101, 102 of the device of fig. 1. It can be seen that the Wells rotor blades 202 are considerably broader than the Darrieus blades 201 (W2 > Wl) . More specifically, the average width of a Wells rotor blade 202 is considerably larger than the average width of the Darrieus blades, wherein the width is calculated from the leading edge (for Wells rotor blade this is leading edge 226) to the trailing edge (for Wells rotor blade this is leading edge 227). This larger width of the Wells blades results in a larger solidity. The effective length of a Wells rotor blade is calculated from the outer circumfer-ence of a central axle 4 to the distal end of the Wells rotor blade, here to the leading edge 226 of the Darrieus rotor blade. The device shown in fig. 2 rotates counter-clockwise.

Fig. 3 shows a variation of fig. 1, wherein the Darrieus blades 301 are positioned obliquely and are connected to the axle 304 as well. This results in a strong construction giving an increased energy output.

Measurements were conducted using a device as shown in fig. 3. It had the following dimensions: - Diameter of the axle 8 cm

Length of the Wells rotor blades including the connecting flange for connecting to the axle: 1.16 m (thus, the overall diameter of the device was 2.40 m) ; However, the connecting flange is not taken into account when calculating σW.

- Distance to the axis of rotation at the lower end of the Darrieus rotor blade: 240 mm

- Angle of the Darrieus blade to the vertical: 30°.

The ratio of the σw and σd was 0.41 / 0.29 = 1.41. In other words, for the device of fig. 3 for conducting the measurements of fig. 4 σw was 41% greater than σd.

In order to reduce the flow resistance at the transitions between the Wells rotor blades and the Darrieus rotor blades these are provided with torpedo shaped bodies 361. For comparison, measurements were also conducted with a similar device without Wells rotor blades (wherein the ends of the Darrieus rotor blades were connected to each other above the waves at the location of the axis of rotation) , and with a device without Darrieus rotor blades. The dimensions of these comparison devices were the same as those of the device according to fig. 3, as indicated above.

Fig. 4a-d display four graphs showing the conducted measurements. The measurements were conducted in a tank having a length of 55 m, 20 m and 20 m, in which artificial waves can be generated having a desired wave height and wave period. In each graph the rotor power coefficient (Cp) is plotted against the tip-speed-ratio (TSR) . In the graphs, W stands for Wells only; D for Darrieus only; WD for a device with both Wells and Darrieus, and W+D for the sum of the curves of W and D. The TSR is the ratio of the velocity of the blade end of the Wells rotor relative to the maximum velocity of the wave at the surface (orbital velocity) . In wind energy the TSR is an often used quantity. There is a difference between waves and wind in that wind has merely 1 component of direction, which substantially has the same magnitude across a turbine blade. Since the velocity at which water moves in a wave varies over time and decreases with the depth, the value for the maximum velocity at the surface has been used for making the graph. The maximum orbital velocity is the distance covered by a water particle at the surface along a vertical circular trajectory having a radius equal to the wave amplitude (= wave height H/2) divided by the wave period Tp; thus 2*pi* (H/2) /Tp = pi*H/Tp (m/s) . In practice, the wave height and period are measured with a wave measuring instrument (eg. Acoustic Wave and Current profiler, wave-rider™ buoy), as known by the person skilled in the art. The TSR can be set at will by increasing or reducing the generator load. The measurements were conducted for four wave periods Tp (I/frequency) . A Tp of 2 means that a wave has a period of 2 seconds (thus, from a first wave top to a second wave top every 2 seconds) .

Fig. 4b-d demonstrate that at a TSR of ca. 4 the measured Cp is larger than the sum of the Cp of the Darrieus rotor and the Cp of the Wells rotor. In case of a short wave period (fig. 4a) this synergy is not observed, but the measured Cp is still higher than that of only a Wells rotor. Therewith, using the device according to the invention achieves a considerable improvement of the efficiency at any time. For the interested layman figures 5 to 8 show the surfaces Ad (swept surface area of a Darrieus rotor) and Aw (swept surface area of a Wells rotor) . Darrieus blades 501, 601, 701, 801, Wells blades 502, 602, 702, 802, axles 504, 604, 704, 804, and generator housings 505, 605, 705 can be seen. In addition, fig. 6 also shows a torpedo shaped body 661, the function of which has already been explained at fig. 3. Fig. 8 clearly shows that in order to determine the effective swept surface area of a Wells rotor, those parts of the Wells blades that lack a Wells profile have to be excluded from consideration. In other words Aw is Awl - Aw2. The effective length of a Wells rotor blade in fig. 8 is the radius of Awl minus the radius of Aw2.